TRANSFORMERS 



389 



weaknesses in the design of a transformer and thus decrease the 

 reliability of operation. 



Induction motors, synchronous motors and synchronous con- 



Per Cent Current 

 20 30 40 50 60 70 



Per Cent Capacity 

 10 15 20 25 M 35 

 Half- Voltage Winding 



FIG. 227. 



100 

 50 



verters started from the A.C. side require frequently transformers 

 with taps for reducing the potential at starting in order to prevent 

 a heavy rush of current. 



Fig. 228 shows the arrangement of taps for starting three-phase 

 converters, leads 1, 2, and 3 being the operating terminals, and 

 leads 1, 4, and 5 those for starting at half voltage. Lead 6 is 

 merely for the purpose of making the three transformers dupli- 

 cates. With some converters it has been found advantageous 

 to insert resistance in the starting connections so as to still further 

 lower the applied voltage. 



Large converters are usually connected six-phase diametrical, 

 and when started from the alternating current side, it has been 

 customary to provide taps on the transformers for one-third and 

 two-thirds voltage, as shown in Fig. 229. Leads 1 to 6, inclusive, 

 are the operating terminals; leads 1, 3, 5, 7, 8, and 9 are for the 



